1. Field of the Invention
The present invention relates to flat display screens. The present invention more specifically applies to screens provided with an internal space (generally under vacuum) isolated from the outside and defined by the spacing between two plates respectively forming the screen bottom and surface.
2. Discussion of the Related Art
Conventionally, a flat screen of the type to which the present invention relates is formed of two generally rectangular spaced apart external plates, for example made of glass. One plate forms the screen surface while the other one forms the screen bottom generally provided with emission means. The two plates are assembled by means of a peripheral seal. For a field-effect screen (FED), or a screen with microtips, or for a vacuum fluorescent display (VFD), vacuum is created in the space separating the two glass plates. In other cases, this space contains a neutral atmosphere (rare gas).
FIG. 1 schematically shows in a cross-section view, the conventional structure of an example of a flat screen of the type to which the present invention relates.
Such a screen is essentially formed, on a first substrate 1, for example made of glass, of an electron bombarding cathode and of one or several grids. In FIG. 1, the cathode/grid(s) assembly is designated by common reference 2. This cathode/grid(s) is placed opposite to a cathodoluminescent anode 3 formed on a second substrate 4, for example made of glass, which is transparent if it forms the screen surface.
An example of a flat screen of the type to which the present invention applies is a microtip screen described, for example, in U.S. Pat. No. 4,940,916 of the Commissariat à l""Energie Atomique.
Cathode/grid(s) 2 and anode 3 are separately formed on the two substrates or plates 1 and 4, which are then assembled by means of a peripheral seal 5. An empty space 6 is created between plates 1 and 4 to enable circulation of the electrons emitted by the cathode to the anode. This space is, in what is designated as its thickness, defined by means of spacers 7 of calibrated height.
The spacers of definition of the inter-electrode space may be formed in several ways.
A first known technique consists of using calibrated balls regularly distributed on one of the plates, the diameter of the used balls (for example, of a given value included between 100 micrometers and 2 millimeters) defines the height of the inter-electrode space. An example of a method for positioning such spherical spacers is described in European patent application No. 0,867,912 of the applicant.
Another known technique for the forming of spacers of definition of the inter-electrode space of a flat screen is to use non-spherical spacers having the shape of posts. These may be sections of cylinders or of posts of various cross-sections (square, rectangular, cross-shaped or others). The use of non-spherical elements is often preferred since it enables minimizing the areas forming obstacles against electron travel between the screen cathode and anode.
The present invention more specifically relates to the placing of non-spherical spacers.
An example of a method for assembling plates of a flat display screen using this type of spacers is described in French patent application No. 2,749,105.
Spacers of non-spherical type are generally positioned and maintained, before fastening (gluing or others), on one of the screen plates, in a grid of small thickness (for example, on the order of from 70 to 90 micrometers). Given its small thickness, such a grid is only proper for spacers of relatively small height (in practice, on the order of 200 micrometers), but no longer enables correct pre-positioning before fastening for spacers of greater height (beyond 400 micrometers). Now, the spacer height that defines the thickness of the inter-electrode space conditions the operating voltage of the flat screen. The higher the desired operating voltage, the thicker the inter-electrode space and the higher the spacers must be.
The grids of positioning and temporary hold of the spacers are generally formed by photoetching techniques, either by electroplating of metal, or to etch a full-plate deposited metal layer, or by etching the very grid.
In the case where the spacers to be positioned have a height greater than 400 micrometers, several layers, generally metallic, must conventionally be superposed.
FIG. 2 illustrates, in a simplified cross-section view, what resembles a superposition of positioning grids. The left-hand portion of FIG. 2 illustrates the superposition of two grids obtained by successive etching of layers 12 deposited full plate, while the right-hand portion of FIG. 2 illustrates the superposition of two grids formed by successive electroplating of pads 11. It should be noted that the superposition of the two grids does not correspond to bringing two grids formed separately one onto the other but to successively performing, on a same substrate (not shown), two electroplating or etching steps.
Whatever the used technique, a mask of definition of openings 10 for positioning spacers 7 or defining pads 11 between the holes distributed in the mask, is used. The mask forming generally uses the deposition of a resist layer. This layer is formed over a thickness generally ranging between 70 and 90 microns. This resist is insolated by means of a lithography mask. Then, the resist is developed by a negative or positive etching according to whether the etching of holes 10 (left-hand portion of FIG. 2) is desired to be obtained or metal (for example, nickel) is desired to be grown around resist pads at the locations of the future holes 10 (right-hand portion of FIG. 2).
A first problem which arises has to do with the thickness desired for the grid. Indeed, with such a thickness, it is not possible to obtain an exposure enabling obtaining an isotropic etching of the holes or of the pads in the resist. Accordingly, as illustrated in FIG. 2, the etching or electroplating is necessarily performed anisotropically and a minimum diameter of holes 10 corresponding to a diameter greater than the diameter (or than the diameter in which the section is inscribed) of spacers 7 must then be provided. For example, for spacers having a cross-section diameter of approximately 50 microns, a minimum diameter of holes 10 on the order of 60 microns must be provided. As a result, the maximum diameter of holes 10 is much greater.
In the case of an electroplating illustrated by the right-hand portion of FIG. 2, the successive layer depositions inevitably come along with an increase of the diameter of holes 10. In the case illustrated in the left-hand portion of FIG. 2, which shows an alternation of steps of full plate deposition of a selectively etchable material 12 and of etching of this material by means of a same exposure mask, the involved thickness inevitably results in anisotropic edges for holes 10.
A first consequence is that the positioning of spacers 7 in the obtained grid has strong risks of occurring incorrectly.
FIGS. 3A and 3B illustrate, in simplified cross-section views of a tool for positioning spacers, an example of implementation of a conventional method for positioning and applying spacers on a flat screen plate.
As illustrated in FIG. 3A, the obtained pre-positioning grid 15 and 15xe2x80x2 (FIG. 2) is laid on a porous or perforated plate 20 of a vacuum table or the like. Plate 20 is generally formed of a porous support of metal or another adapted material (ceramic, etc.). Space 22 underlying plate 20 is closed by an enclosure 21 partially shown and this space 22 communicates with a pumping opening 23 connected to a vacuum pump (not shown). The suction caused by the pump on plate 20 is transmitted by holes 10. In a simplified embodiment, a significant volume of spacers 7 is just roughly distributed on the surface of pre-positioning grid 15 or 15xe2x80x2, after which the vacuum pump is operated so that a spacer 7 is retained in each hole 10 after having entered therein by suction. The excess spacers can then be eliminated, for example, by turning the tool upside down above a recovery tank, or by sweeping, blowing, vibration, inclined plane, etc.
As illustrated in FIG. 3A, in the case of a grid 15 manufactured by electroplating, there is a non-negligible risk of seeing some spacers be placed completely slantwise in holes 10. This phenomenon is not as strong in the case of a grid 15xe2x80x2 obtained by full plate deposition and etching of different layers but however remains, mainly due to the difficulty of perfectly aligning the mask upon insolation prior to the etching of the different levels. The hole of a higher level will generally have a diameter greater than that of a hole of lower level, or shifted with respect thereto.
Once the spacers are individually maintained in the respective holes 10 of the pre-positioning grids, a plate coated with glue is brought onto the free ends of spacers 7 so that a thin layer of glue 16 deposits thereon. Finally, as illustrated in FIG. 3B, the screen plate (for example, 1) on which the spacers are desired to be glued is brought and applied on the free ends, now sticky, of spacers 7 which are thus maintained thereon. Once the fastening has been performed, the vacuum is cut-off in the vacuum table, which frees the spacers from the pre-positioning grids.
The rest of the flat screen assembly method is perfectly conventional and will not be detailed herein. It should only be reminded that the second screen plate (for example, 4) is added to be parallel to the first one with an interposed peripheral seal 5 as illustrated in FIG. 1.
Another problem that is posed in the positioning of the spacers on a screen plate is, independently from height problems, linked to the indispensable tolerances to be provided between the diameter of the positioning grid holes and the cross-section diameter of the spacers. Indeed, a rigorously adapted diameter cannot be provided. Now, to limit the obstacles to the electron travel between the cathode and the anode, as exact a positioning of the spacers on areas of no electron emission as possible must be searched. In practice, it is desired to arrange these spacers between the screen pixels generally defined by the intersection between cathode columns and lines of the associated extraction grid.
Above-mentioned French patent application NO. 2,749,105 provides different solutions of pre-positioning grid superposition to attempt reducing the above disadvantages. According to a solution of this document, it is provided to interpose a thick grid (210 micrometers) between two relatively thin grids (70 micrometers) which are made with more precision than this thick grid. However, the non-isotropic character of the holes in the external layers of the grid is nevertheless present due to the thickness of this grid. Further, this solution does not solve the necessary tolerance problem linked to the introduction of the spacers into the holes, which adversely affects the accurate positioning of these spacers on the screen plate.
The present invention aims at overcoming the disadvantages of known solutions for spacer pre-positioning grids between two screen plates to be assembled.
The present invention more specifically aims at providing a novel tool enabling avoiding all risks of spacer inclination upon installation.
The present invention also aims at providing a solution which optimizes the alignment of the free ends of the different spacers.
The present invention also aims at providing a novel spacer placing method which improves the positioning accuracy of these spacers on the screen plate. On this regard, the present invention also aims at providing a tool adapted to such a method.
The present invention further aims at easing the handling of the spacer positioning tool.
To achieve these objects, the present invention provides a tool for positioning spacers on a first plate intended for being maintained at a distance from a second plate by said spacers, said tool including openings for receiving said spacers, and said openings being of variable size between a first position of introduction of the spacers and a second position of mechanical blocking of the spacers.
According to an embodiment of the present invention, the general thickness of the positioning tool is smaller than one third of the height of the spacers.
According to an embodiment of the present invention, said openings have, in the first position, a diameter greater than the diameter in which the section of a spacer is inscribed, smaller than the height of the spacer and such that two spacers cannot be introduced therein at the same time.
According to an embodiment of the present invention, the positioning tool includes at least two grids in planes parallel to each other, at least one first grid being assembled to slide parallel to a second grid.
According to an embodiment of the present invention, the positioning tool includes two external grids attached in planes parallel to each other to define the distribution of the spacers, and at least one grid for locking the spacers in their position, slidably assembled between said two external grids.
According to an embodiment of the present invention, said two external grids include holes having a diameter substantially greater than the diameter in which the section of the spacers to be positioned is inscribed.
According to an embodiment of the present invention, said two external grids include holes of same diameter.
According to an embodiment of the present invention, said locking grid includes holes having a diameter at least equal to the diameter of the holes of the external grids.
According to an embodiment of the present invention, the thickness of the external grids is chosen according to the maximum tolerance desired for the positioning of the spacers.
According to an embodiment of the present invention, the thickness of the external grids is smaller than 50 micrometers.
According to an embodiment of the present invention, the holes of at least one locking grid are each associated with a resilient tab for blocking a spacer in its position.
According to an embodiment of the present invention, the holes of at least one of the external grids each include a notch for receiving one end of an arm of a spacer, said spacers having, in cross-section, the shape of a cross.
According to an embodiment of the present invention, the positioning tool includes at least one ductile grid provided with holes at least at the locations of the spacers, a change of size of said holes being caused by a controlled reversible deformation of this grid.
According to an embodiment of the present invention, the positioning tool includes at least one rigid grid parallel to the ductile grid and provided with holes approximately aligned with those of the ductile grid when said grid is in a first position.
The present invention also provides a spacer positioning method, consisting of using a vacuum table for placing a spacer in each opening of the positioning tool in a first position, then performing successive suction and blowing cycles, by applying a free end of the spacers against an alignment plate, before their locking in their position by narrowing of the openings.